High-purity nitrogen gas production equipment

ABSTRACT

There is disclosed a high-purity nitrogen gas production equipment for production of ultra-high-purity nitrogen gas for use in the electronics and other industries, for example in connection with the production of silicon semiconductors. The conventional nitrogen gas production equipment of cryogenic air separation type tends to develop troubles and yields product nitrogen gas only at high cost and in comparatively low purity. The equipment according to the invention is such that a liquid nitrogen storage means (23) is connected via a feeding pipeline (24a) to a column segment (22) of a distillation column (15) which consists of a partial condenser segment (21) having a built-in condenser (21a) and the intermediate-pressure column segment (22) and the cryogenic compressed air supplied into the intermediate-pressure column segment (22) of the distillation column (15) via an air compression means (9) and heat exchange means (13), (14) is further chilled by the liquid nitrogen reflux liquid obtained in said partial condenser segment (21) and the heat of evaporation of the liquid nitrogen supplied from the liquid nitrogen storage means (23). By taking advantage of the difference in boiling point, the nitrogen is withdrawn as an intermediate-pressure gas from a top portion of the column segment (22) while oxygen is retained in liquid state. The resulting intermediate-pressure nitrogen gas is used as product nitrogen gas. By these features, low-cost, high-purity nitrogen gas can be produced without an energy loss due to pressure loss and without machine troubles.

TECHNICAL FIELD

The present invention relates to a production equipment for high-puritynitrogen gas.

BACKGROUND ART

While the electronics industry consumes a very large quantity ofnitrogen gas, stringent requirements have been imposed on the purity ofthe nitrogen gas they use from the standpoint of maintenance of the highprecision of parts. Nitrogen gas is generally produced from air in aproduction sequence which consists of compressing the air with acompressor, passing the compressed air through an adsorbent column toremove carbon dioxide gas and water, feeding the emerging air further toa heat exchanger where it is chilled by heat exchange with arefrigerant, feeding the chilled air to a distillation column forcryogenic liquefaction and separation to give product nitrogen gas, andfinally passing the same through said heat exchanger to heat it up to atemperature near atmospheric temperature. However, the product nitrogengas thus produced contains oxygen as an impurity and the use of thisnitrogen gas as it is presents various prcblems. One of the methods forremoving impurity oxygen (1) comprises adding a small amount of hydrogento nitrogen gas and reacting the hydrogen in the mixture with theimpurity oxygen in the nitrogen gas in the presence of a platinumcatalyst at a temperature of about 200° C. to remove the impurity oxygenin the form of water. Another method (2) comprises contacting nitrogengas with a nickel catalyst at a temperature of about 200° C. to removethe impurity oxygen by way of the reaction Ni +1/2O₂ →NiO. However, asboth methods involve the step of heating nitrogen gas to a hightemperature for catalytic reaction, the corresponding hardware cannot bebuilt into the nitrogen gas production line which is a cryogenic system.That is to say, the purification equipment must be installedindependently of the nitrogen gas production equipment and this entails,of necessity, the disadvantage that the overall size of the productionplant is increased. Furthermore, the first-mentioned method (1) requiresexact control over the addition level of hydrogen. Unless hydrogen isadded in an amount exactly commensurate with the amount of impurityoxygen present, either some oxygen remains in the product gas or thevery hydrogen so added becomes a new impurity, so that high skill isrequired in operation. In the second-mentioned method (2), the NiOproduced in the reaction with impurity oxygen must be regenerated(NiO+H₂ →Ni+H₂ O) and the cost of the H₂ gas equipment for catalystregeneration contributes to an increased purification cost. Solutions tothese problems have been awaited.

Furthermore, the conventional nitrogen gas production equipment employsan expansion turbine for chilling the refrigerant used for heat exchangewith compressed air from the compressor and this turbine is driven bythe pressure of the gas generated by gasification of the liquid aircollecting in the distillation column (as the result of cryogenicliquefaction and separation, the low-boiling nitrogen leaves the column,while the balance in the form of an oxygen-rich liquid air collects inthe column). However, the expansion turbine has a high rotational speed(in the order of tens of thausands of revolutions per minute) and cannoteasily follow a variation in load, thus requiring a specially trainedoperator. Moreover, as a high-speed machine, the expansion turbine notonly demands high-precision in construction and is costly but requiresspecially trained personnel for its operation. These problems emanateall from the high-speed rotary mechanism of the expansion turbine andthere has been a strong demand for elimination of the expansion turbinehaving such a high-speed rotary mechanism.

OBJECT OF THE INVENTION

It is an object of the present invention to provide a high-puritynitrogen gas production equipment which requires neither an expansionturbine nor a purification system.

DISCLOSURE OF THE INVENTION

Developed for the purpose of accomplishing the above-mentioned object,the present invention comprises an air compression means for compressingthe air from an external environment, an elimination means foreliminating carbon dioxide gas and water from the compressed air, a heatexchange means for chilling the compressed air from said eliminationmeans to a cryogenic temperature, a distillation column adapted toliquefy a portior of the cryogenic compressed air from said heatexchange means and collect the same therein while retaining nitrogenalone in gaseous form, a liquid nitrogen storage means for storingliquid nitrogen, a feeding pipeline for leading liquid nitrogen in saidliquid nitrogen storage means to said distillation column for use as arefrigerant, and a nitrogen gas withdrawal line for withdrawing theretained gaseous nitrogen from said distillation column, saiddistillation column consisting of a partial condenser segment having abuilt-in condenser for production of reflux liquid and a column segmentfor liquefaction and separation of compressed air, said partialcondenser segment communicating with the bottom of said column segmentvia a liquid air intake pipeline equipped with an expansion valve andthe inlet and outlet of said built-in condenser in said partialcondenser segment communicating with the top of said column segment viaa first and a second reflux pipeline, respectively, and said columnsegment being connected at its bottom to said heat exchange means and atits top to said feeding pipeline and nitrogen gas withdrawal line.

EFFECTS OF THE INVENTION

The high-purity nitrogen gas production equipment according to thepresent invention does not employ an expansion turbine but, instead,employs a liquid nitrogen storage means such as a liquid nitrogenstorage tank having no rotary element and, therefore, the wholeequipment has no revolving parts and, hence, is trouble-free.Furthermore, whereas the expansion turbine is costly, the liquidnitrogen tank is not expensive and does not require special personnelfor operation. In addition, the expansion turbine (which is driven bythe pressure of the gas generated from the liquefied air collectedwithin the nitrogen distillation column) is driven at a very high speed(the order of several times a thousand revolutions per minute), it isdifficult to follow a delicate variation in load (the variation in therate of withdrawal of product nitrogen gas). Therefore, it is difficultto accurately vary the supply of liquefied air to the expansion turbineaccording to the change in the outgoing product nitrogen gas so as tochill the compressed air, which is the raw material for nitrogen gas, toa constant temperature at all times. As a consequence, the productnitrogen gas varies in purity so that low-purity products may bewithdrawn from time to time to affect the overall quality of production.

In contrast, as the equipment according to the present invention employsa liquid nitrogen storage tank, in lieu of the expansion turbine, andliquid nitrogen, which permits delicate control of feed, as arefrigerant, the equipment allows for delicate followup of loadvariation and, thus, enables one to produce nitrogen gas of extremelyhigh and uniform purity. This, in turn, erables one to dispense with thepurification system heretofore required. Furthermore, the equipmentaccording to the present invention comprises a partial condenser segmenthaving a built-in condenser for production of reflux liquid and a columnsegment for liquefaction and separation of compressed air, and thecolumn segment is supplied with the compressed air prepared by an aircompression means substantially without a pressure loss. As a result,product nitrogen gas is produced substantially without a loss of energyand, hence, the cost of product nitrogen gas is reduced. In addition, asthe pressure of the product nitrogen gas is high, a larger quantity ofgas can be transported with pipelines of a given diameter and assumingthat the transport quantity is kept constant, pipes of smaller diametercan be employed so as to effect economies in the initial cost of theequipment.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic process diagram showing one embodiment of thepresent invention;

FIG. 2 is a schematic process diagram showing a modification thereof;and

FIG. 3 is a schematic process diagram showing still another embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be described in detail with reference to itsembodiments

FIG. 1 shows an embodiment of the present invention. In FIG. 1, thereference numeral 9 indicates an air compressor, 10 a drain separator,11 a Freon refrigerator, and 12 a couple of adsorbent columns. Eachadsorbent column is packed with a molecular sieve which adsorbs andremove H₂ O and CO₂ from the compressed air from said air compressor 9.Indicated at 8 is a compressed air pipeline for feeding the compressedair freed of H₂ O and CO₂ by adsorption. The numeral 13 indicates afirst heat exchanger which is supplied with the compressed air freed ofH₂ O and CO₂ in the adsorbent column couple 12. To a second heatexchanger 14 is fed the compressed air from the first heat exchanger 13.The numeral 15 indicates a distillation column, the top portion of whichconstitutes a partial condenser segment 21 having a condenser 21a, withthe underneath portion constituting a column segment 22. In thedistillation column, the compressed air chilled to a cryogenictemperature in the first and second heat exchangers 13, 14 and fed viathe pipeline 17 is further chilled and a portion thereof is liquefiedand collects in the bottom of the column segment 22 as liquefied air 18while nitrogen alone is pooled in gaseous state in the top ceilingportion of the column segment 22. A liquid nitrogen storage tank 23contains liquid nitrogen (high-purity product) which is fed via afeeding pipeline 24a into the top of the column segment 22 of thedistillation column 15 for use as a refrigerant for the compressed airintroduced into the column segment 22. The above-mentioned distillationcolumn 15 is now described in detail. The distillation column 15 isdivided by a partitioning plate 20 into the partial condenser segment 21and the column segment 22, and the condenser 21a in the partialcondenser segment 21 is supplied with a portion of the nitrogen gascollected in the top portion of the column segment 22 via a pipeline21b. The inside of this partial condenser segment 21 is relativelydecompressed with respect to the inside of the column segment 22, andthe liquefied air (N₂, 50-70%; O₂, 30-50%) pooled in the bottom of thecolumn segment 22 is fed via a pipeline 19 equipped with an expansionvalve 19a and gasified therein to lower the internal temperature to alevel below the boiling point of liquid nitrogen. As a result of thischilling, the nitrogen gas fed into the condenser 21a is liquefied. Thenumeral 25 indicates a evel gauge. According to the level of liquefiedair in the partial condenser segment 21, a valve 26 is controlled toadjust the supply of nitrogen gas from a liquid nitrogen storage tank23. The top portion of the column segment 22 of the distillation column15 is supplied with the liquid nitrogen produced in the condenser 21a ofsaid partial condenser segment 21 via a down-coming pipeline 21c andalso with liquid nitrogen from the liquid nitrogen storage tank 23 viathe pipeline 24a. These two streams of liquid nitrogen flow down thecolumn segment 22 from a liquid nitrogen basin 21d and come incounter-current contact with, and cool, the compressed air ascendingfrom the bottom of the column segment 22 to thereby liquefy part of thecompressed air. In this process, the high-boiling components in thecompressed air are liquefied and collect in the bottom of the columnsegment 22, while nitrogen gas which is a low-boiling component collectsin the top portion of the column segment 22. The reference numeral 27indicates a withdrawal pipeline for withdrawing the nitrogen gas cooledin the top ceiling portion of the column segment 22 of the distillationcolumn as product nitrogen gas. This pipeline guides the cryogenicnitrogen gas to the second and first exchangers 14, 13 for heat exchangewith the compressed air fed thereto, and leads it at atmospherictemperature to a main pipeline 28. In this connection, since low-boilingHe (-269° C.) and H₂ (-253° C.) tend to collect, together with nitrogengas, in the uppermost portion of the column segment 22 of thedistillation column, the withdrawal pipeline 27 is disposed tocommunicate at a substantial distance below the uppermost portion of thecolumn segment 22 so that pure nitrogen gas free from He and H2 may bewithdrawn as product nitrogen gas. The reference numeral 29 indicates apipeline for feeding gasified liquid air in the partial condensersegment 21 to the second and first heat exchangers 14, 13, with apressure control valve thereof being indicated at 29a. The numeral 30indicates a backup system line which, in the event of a failure of theair compression line, evaporates the liquid nitrogen in the liquidnitrogen storage tank 23 by means of an evaporator 31 and feeds it tothe main pipeline 28 so as to prevent interruption of nitrogen gassupply. Indicated at 32 is an impurity analyzer which analyzes thepurity of product nitrogen gas going out into the main pipeline 28 and,when the purity is low, actuates valves 34 and 34a to let off theproduct nitrogen gas in the direction indicated by the arrowmark B.Further, blowoff conduit 21e blows off gasified liquid air produced inthe partial condenser segment to the outside.

The equipment described above produces product nitrogen gas in thefollowing manner. Thus, the air compressor 9 compresses the material airand the drain separator 10 removes water from the compressed air. Thefreon refrigerator 11 chills the compressed air and the chilled air isfed to the adsorption columns 12, where H₂ O and CO₂ in the air areadsorbed and removed. This compressed air freed of H₂ O and CO₂ is fedto the first and second heat exchangers 13, 14 which have been cooled bythe product nitrogen gas, etc. supplied from the distillation column 15via the pipeline 27, where it is chilled to a cryogenic temperature. Thechilled air is then directly charged into a lower portion of the columnsegment 22 of the distillation column. This charged compressed air ischilled by contact with the liquid nitrogen fed into the column segment22 from the liquid nitrogen storage tank 23 via the feeding pipeline 24aand the liquid nitrogen overflowing the liquid nitrogen basin 21d,whereby a portion of the air is liquefied and collects as liquid air 18in the bottom of the column segment 22. In this process, due to thedifference between nitrogen and oxygen in boiling point (boiling pointof oxygen -183° C.; boiling point of nitrogen -196° C.), oxygen which isa high-boiling fraction in the compressed air is liquefied whilenitrogen remains as a gas. Then, this remaining gaseous nitrogen iswithdrawn through the withdrawal pipeline 27 and fed to the second andfirst heat exchangers 14, 13, where it is heated to a temperature nearatmospheric temperature. This nitrogen is withdrawn from the main pipe28 as product nitrogen gas. In this connection, as the inside of thecolumn segment 22 of the distillation column is held at a high pressureowing to the compressive force of the air compressor 9 and the vaporpressure of liquid nitrogen, the pressure of product nitrogen gas takenout from the withdrawal pipeline 27 is also high. This is advantageouswhen the product nitrogen gas is used as a purge gas. Moreover, becauseof this high pressure, a larger quantity of gas can be transported withpipelines of a given diameter and assuming that the amount oftransportation is constant, pipes of smaller diameter can be utilized sothat the equipment cost may be decreased. On the other hand, theliquefied air 18 collected in the lower part of the column segment 22 ofthe distillation column is fed into the partial condensor segment 21where it is used to cool the condenser 21a. By this cooling, thenitrogen gas fed into the condenser 21a from the top portion of thecolumn segment 22 of the distillation column is liquefied to form areflux within the column segment 22 and recycled to the column segment22 via the pipeline 21c. And the liquefied air 18 which has cooled thecondensor 21a is gasified and flows to the secord and first heatexchangers 14, 13 via the pipeline 29 to chill the heat exchangers 14,3, after which it is exhausted into the atmosphere. The liquid nitrogenfed from the liquid nitrogen storage tank 23 into the column segment 22of the distillation column via the feeding pipeline 24a functions as arefrigerant for the liquefaction of compressed air and is gasified andwithdrawn from the withdrawal pipeline 27 as part of product nitrogengas. In this manner, the liquid nitrogen in the liquid nitrogen storagetank 23, after discharging its function as a refrigerant forliquefaction of compressed air, is not discarded but is combined withthe high-purity nitrogen gas made from compressed air as productnitrogen, so that wasteless utilization can be realized.

In FIG. 2 is shown an embodiment wherein a vacuum cold housing isadditionally provided in the equipment of FIG. 1. Thus, in thisembodiment, the distillation column 15 and the first and second heatexchangers 13, 14 are accommodated in a vacuum cold housing (indicatedin dot-dash line) for enhancement of distillation efficiency. Otherwise,this equipment is identical with the equipment illustrated in FIG. 1.

FIG. 3 shows an embodiment wherein a condenser is provided within thecolumn segment of the nitrogen distillation column of the equipmentshown in FIG. 1. Thus, in this equipment, a condenser 22a is providedwithin the column segment 22 of the nitrogen distillation column 15 andthe liquid nitrogen in the liquid nitrogen storage tank 23 is fed as arefrigerant via the feeding pipeline 24a to the above condenser to chillthe compressed air supplied from the lower portion of the column segment22 and ascending up the column segment 22 to thereby liquefyhigh-boiling fractions such as oxygen and collect them in the bottom ofthe column segment 22, while nitrogen gas which is low-boiling collectsin the top portion of the column segment 22. And the gasified liquidnitrogen after functioning as a refrigerant in the condenser 22a isguided to the withdrawal pipeline 24b, subjected to heat exchange in thesecond and first heat exchangers 14, 13, and discharged from the system.Otherwise, this equipment is identical with the equipment of FIG. 1.

I claim:
 1. An apparatus for producing highly pure nitrogen gascomprising a means to compress the air taken from the outside, a meansto remove carbon dioxide gas and water from the compressed air from saidair compression means, a heat exchange means to chill the compressed airfrom said removing means to a cryogenic temperature, a rectificationcolumn to liquify a portion of the cryogenic compressed air from saidheat exchange means and collect it in the lower region of the columnwhile taking out pure nitrogen gas from the tip portion, wherein theapparatus further comprises a partial condenser segment having acondenser therin at the upper end of the rectification column, a feedingconduit to lead an accumlating liquified air in the lower region of therectification column into the partial condenser segment for use as arefrigerant to cool said condenser, a blowoff conduit to blow offgasified liquid air produced in the partial condenser segment to theoutside, a first relux liquid conduit to lead part of the nitrogen gasproduced in the rectification column into said condenser, a secondreflux liquid conduit to return the liquified nitrogen produced in thecondenser to the rectification column as a reflux, a liquid nitrogenstorage means to store liquid nitrogen supplied from outside theapparatus, a leading channel to lead the lquid nitrogen in the liquidnitrogen storage means to the column, a means to control the level ofthe liquid air in the condenser by controlling the supply of liquidnitrogen from the nitrogen storage means to the column, and an outletchannel to take out both the nitrogen gas from the rectification columnand the gasified nitrogen in the recitification column after use as acooling source, and to pass them through said heat exchanger and heatthem by exchanging heat with the compressed air.
 2. The nitrogen gasproduction equipment of claim 1, wherein the distillation column and theheat exchange means are disposed within a vacuum cold housing.
 3. Thenitrogen gas production equipment of claim 1, wherein a second condenseris disposed within the column segment to receive liquid nitrogen fromthe liquid nitrogen storage means so as to chill compressed air from thelower portion of the column segment.